A laser device generates coherent radiation with a wavelength fixed to the atomic transition between quantum energy levels of a laser gain medium. The pump source to a laser device can be a broadband flash lamp or a narrow-band diode laser, which provides pump radiation with its photon energy matched to the absorption transition levels of the laser gain medium. Usually the laser gain medium is contained in a laser cavity with a laser axis defined by two end mirrors. In the case of a laser amplifier, a seed laser beam enters the laser gain medium and defines the laser axis. For a diode pumped laser, there could be a pump beam direction. The laser axis relative to the pump beam direction is not limited by fundamental laser physics, but mostly determined by considerations such as overlapping efficiency with the gain medium, thermal dissipation, and hardware arrangement etc. For instance, U.S. Pat. No. 6,268,956 B1 taught an end diode-pumped laser with a zigzag laser axis in a laser crystal to increase the overlap between the pump absorption and the generated laser radiations or help thermal dissipation. Modifying the zigzag angle with respect the pump beam geometry does not change the laser wavelength or modify the coherence property of the output laser.
A nonlinear laser frequency converter is technically different from the aforementioned laser source. A nonlinear optical material is useful for converting a laser frequency to a different one without relying on quantum energy levels in the material. A nonlinear process splitting one high-frequency input photon into several low-frequency output ones is called frequency down-conversion or parametric generation, whereas its inverse process is called frequency up-conversion or sum frequency generation. For the up-conversion process, if all the low-frequency input photons have the same frequency, the process is called harmonic generation. Unlike a laser having its input or pump wave absorbed in the gain medium to emit an output wave, a nonlinear laser-frequency converter has several co-propagating waves, which are not supposed to be absorbed in the nonlinear optical gain medium. For the nonlinear frequency conversion to happen, fundamental physics imposes constraints on the relative directions of the mixing laser beams. For instance, the nonlinear laser-wavelength conversion in a quadratic nonlinear optical material permits a 3-photon or 3-wave mixing process satisfying the frequency ruleω3=ω1+ω2,  (1)and the wave-vector rule{right arrow over (k)}3={right arrow over (k)}1+{right arrow over (k)}2,  (2)where ωi is the angular frequency and {right arrow over (k)}i is the wave vector of the ith wave. When multiplied by the reduced Planck constant, =h/2π, Eqs. (1,2) become conservations of photon energy and momentum. Equation (2) is often called the phase matching condition, because the multiplication of a wave-vector with a position vector gives the radiation phase. In a nonlinear mixing process, the output radiation, if any, automatically satisfies the frequency rule or photon energy conservation. However, the wave-vector rule is relevant to material dispersion and the propagation directions of the mixing waves. Ideally, collinear phase matching (k3−k1−k2=0), where all the wave vectors are aligned collinearly in the gain medium, maximizes overlap among mixing waves and gives the best laser-frequency conversion efficiency. Unfortunately, only a small number of nonlinear optical materials have adequate dispersion for collinear phase matching over a limited spectral band width. In most collinear nonlinear wave mixing cases, the nonlinear laser frequency conversion only occurs in a dephasing distance or a so-called coherence length, given by
                              L          c                =                  π                      Δ            ⁢                                                  ⁢            k                                              (        3        )            where Δk=|k3−k1−k2|. To overcome such a difficulty, U.S. Pat. No. 5,640,480 taught a phase matching scheme that requires all the collinearly propagating mixing waves to zigzag through a slab nonlinear crystal with a thickness on the order of the coherence length Lc, so that the phase mismatch can be compensated by the reflection phase at the crystal boundaries. For a thick crystal, this scheme becomes ineffective, as the co propagating waves between reflections remain phase mismatched for a thick distance until reaching the reflection boundaries.
On the other hand, nonlinear laser frequency conversion with a non-collinear phase matching configuration often suffers from poor overlap among mixing waves and thus poor conversion efficiency. FIG. 1 illustrates a resonantly enhanced, non-collinearly phase matched, nonlinear laser-frequency down-converter or a parametric oscillator in the prior art, wherein a laser source 100 generating a pump laser beam 110 with frequency ω3 is incident on a piece of quadratic nonlinear optical material 115 to generate the first frequency down converted radiation wave 125 with frequency ω1 and the second frequency down converted wave 120 with frequency ω2. The three frequencies satisfy the frequency rule Eq. (1), 130. Assuming ω3>ω2>ω1, the high frequency output wave at ω2 is usually called the signal and the low frequency output wave at ω1 is called the idler. Material dispersion and the phase matching condition in Eq. (2) determine the relative propagation angles of the three mixing waves, θ and ψ, as shown in the wave-vector relationship in the dashed box 135. It is evident that the overlap among the three mixing waves is relatively poor when compared with the case with a collinearly phase matching configuration. Any of the waves walking off from the other two terminates energy coupling among the three and degrades the laser conversion efficiency. To increase the laser conversion efficiency, a resonator comprising two reflecting mirrors 140 and 145 can be built to oscillate and amplify the second parametrically generated wave 120. The first parametrically generated wave 125 is coupled out from an output coupler 150. The output coupled can be an index-matching prism atop the nonlinear optical material 115. Such a laser frequency down converter resonating only one output wave is called a singly resonant optical parametric oscillator (SRO). For a resonator with an optical length L, the roundtrip phase of the resonant wave satisfies the longitudinal standing wave condition2kmL+2φ=2mπ,m=1,2,3.  (4)where km is the wave number of the mth longitudinal radiation mode and φ is the reflection phase on the resonator mirror. Here, without loss of generality, the reflection phases from the two mirrors are assumed to be the same, equal to φ. As a result, the longitudinal-mode frequencies of the resonant radiation wave 120 are given byω2,m=mωfsr,  (5)where ωfsr=2π×c/2L is the free spectral range of the resonator and c is the speed of in the resonator. When a second set of cavity mirrors is also inserted to oscillate the other output wave 125, the radiation device is called a doubly resonant optical parametric oscillator (DRO). If the cavity mirrors 140 and 145 are both removed and either the first or the second parametrically generated wave 125 or 120 is seeded by an external radiation source, the radiation device is called an optical parametric amplifier (OPA) or difference frequency generator (DFG).
A Tera-Hertz (THz) wave is usually referred to an electromagnetic wave having a frequency between 1011 (0.1 THz) and 1013 Hz (10 THz). The parametric radiation source depicted in FIG. 1 is also widely adopted for generating THz wave radiation via the so-called stimulated polariton scattering (SPS) in some polar nonlinear optical material with parametric gain. For SPS with reference to FIG. 1, the THz wave (idler wave) 125 often emits with a large angle ψ and the red-shifted signal wave 120 emits with a much smaller angle θ from an optical pump beam 110. The THz parametric oscillator (TPO) in FIG. 1 has been detailed by Kawase et al. in the review article “Terahertz wave parametric source” J. Phys. D: Appl. Phys. 34 (2001) R1-R14. When the slightly red shifted signal wave 120 is externally seeded, the radiation device is called a THz parametric amplifier (TPA). Popular nonlinear polar materials for TPO and TPA include lithium niobate (LiNbO3, LN), lithium tantalate (LiTaO3, LT), Potassium titanyl phosphate (KTiOPO4, KTP), Potassium Titanyle Arsenate (KTiOAsO4, KTA), which are all highly absorptive at THz frequencies. Therefore, the aforementioned TPO or TPA in the prior art suffers from extremely poor THz radiation efficiency due to both the non-collinear phase matching and strong THz absorption in the material. How to effectively and efficiently generate the THz wave via SPS in a non-collinearly matched nonlinear optical material is still under development in the art.
All the non-collinearly phase matched parametric radiation devices suffer from poor overlap among mixing waves and thus poor conversion efficiency due to one or several problems in walkoff, diffraction, and absorption of the mixing waves. Therefore, it is an intention of the present invention to provide a new parametric radiation device and its embodiments to overcome the above-mentioned drawbacks in the prior art.